Question – What do you understand by atmosphere? Discuss the composition and structure of earth’s atmosphere. – 1 December 2021
Answer – The envelope of gases surrounding the earth is called the atmosphere. It forms a protective boundary between the outer space and the biosphere. It is a mixture of gases that is odorless, colorless, tasteless and formless mixed and blended so thoroughly that it acts as a single gas.
All life forms need a particular range of temperature and a specific range of frequencies of solar radiation to carry out their biophysical processes. The atmosphere absorbs certain frequencies and lets through some other frequencies of solar radiation. In other words, the atmosphere regulates the entry of solar radiation. It protects the earth from the harmful radiation from the sun. The atmosphere also keeps the temperature over the earth’s surface within certain limits. In the absence of the atmosphere extremes of temperature would exist between day and night over the earth’s surface.
Composition of the atmosphere:
The atmosphere is composed of –
The atmosphere is a mixture of many gases. In addition, it contains huge numbers of solid and liquid particles, collectively called aerosols.
- Nitrogen accounts for 78% of total atmospheric volume. It is a relatively inert gas, and is an important constituent of all organic compounds. The main function of nitrogen is to control combustion by diluting oxygen.
- Oxygen, although constituting only 21% of the total volume of the atmosphere, is the most important component among gases. All living organisms require oxygen for survival.
- Carbon Dioxide which constitutes only about 0.038% of the dry air. It is transparent to the incoming solar radiation (insolation) but opaque to the outgoing terrestrial radiation. Carbon dioxide is largely responsible for the greenhouse effect. It is also an essential component of photosynthesis.
- Argon is the third important gas in the atmosphere which constitutes only about 0.93%.
- Ozone is another important component of the atmosphere found mainly between 10 and 50 km above the earth’s surface. It acts as a filter and absorbs the ultra-violet rays radiating from the sun and prevents them from reaching the surface of the earth.
- Other gases found in almost negligible quantities in the atmosphere are neon, helium, hydrogen, xenon, krypton, methane, etc.
Water Vapour: The vapour content in the atmosphere ranges from 0 to 5 % by volume. The atmospheric vapour is received through the evaporation of moisture and water from the water bodies (like seas and oceans, lakes, tanks and ponds, rivers, etc.), vegetation and soil cover.
Particulate Matter: The Particulate Matter present in the atmosphere consist of sand particles (from weathered rocks and also derived from volcanic ash), pollen grains, small organisms, soot, ocean salts; the upper layers of the atmosphere may even have fragments of meteors which got burnt up in the atmosphere.
Structure of the Atmosphere:
The atmosphere can be divided into five layers according to the variation of temperature and density:
(a) Troposphere: This is the lowest layer of the atmosphere. It extends from 18 km on the equator to 8 km on the poles. This is the most important layer of the atmosphere because all kinds of weather changes take place only in this layer. The environmental temperature decreases with increasing height of the atmosphere. It decreases at the rate of 10C at a height of 165 meters. The air never remains static in this layer. The upper limit of the troposphere is called the tropopause.
(b) Stratosphere: This layer is spread up to the height of 50 km from the Earth’s surface. Its average extent is 40 km. Here temperature increases slowly with the increase in height. The temperature increases due to the presence of ozone gas in the upper part of this layer.
(c) Mesosphere: It is the third layer of the atmosphere spreading over stratosphere. It spreads up to the height of 80 km from the surface of the earth. Temperature goes on decreasing and drops up to – 1000C. ‘Meteors’ or falling stars occur in this layer.
(d) Ionosphere: This is the fourth layer of the atmosphere. It is located above the mesosphere. This layer spreads up to a height of 400 km from the surface of the earth. This layer is electrically charged and because of the electric charge, radio waves transmitted from the earth are reflected back to the earth by this layer.
(e) Exosphere: This is the outer most layer of the atmosphere located above ionosphere and extends to beyond 400 km above the earth. Gases are very sparse in this sphere due to the lack of gravitational force. Therefore, the density of air is very less here